An oxygen senor monitors the ratio of oxygen to gasoline in a vehicle. If there is less oxygen present in the ratio, fuel will remain after combustion. This results in a rich mixture. Rich mixtures create pollution due to the remaining unburned fuel. If there is an excess of oxygen present in the ratio a lean mixture exists. Lean mixtures also tend to produce pollutants. Moreover, lean mixtures may cause poor engine performance and possibly engine damage. In determining whether a rich or lean mixture is present, the oxygen sensor undergoes a chemical reaction that generates a voltage. In one embodiment of a known sensor, rich mixtures have a voltage reading in the range of 450 millivolts (mV) or higher, while lean mixtures have a reading in the range of 449 millivolts or lower. In ideal operating conditions, the oxygen sensor should have an average high and low reading of 450 millivolts at idle or at steady cruise operations.
The oxygen sensor is part of the emissions control system. Positioned in the exhaust system to sample exhaust, it typically is connected to feed data to a powertrain control module (PCM). The data received from the oxygen sensor provides the PCM with input that may be used to calculate the efficiency of the cylinder burn process. To protect the integrity of the catalytic converter and to provide the greatest amount of vehicle performance, the PCM constantly adjusts fuel delivery to the cylinders based upon the input from the oxygen sensor. The PCM expects the oxygen sensor to pass the median voltage a certain number of times and within a pre-determined period. Thus, oxygen sensors may be a source of information regarding the vehicle's performance. However, there is no standardized or automated process for verifying the operation of the oxygen sensor during diagnostic testing. As a result, working oxygen sensors may be replaced during service because the technician may not properly conduct diagnostic testing on the oxygen sensor.